How do i partition my 1TB HD which has just one volume ?

And yes, for any reason we always have a backup of what we can't lose. Hard drives fail all the time and some folk are not prepared.

http://www.ubuntu.com/download/help/install-desktop-latest does not go into great detail but the last time I did that I did not have to partition the drive. I select the option to install Ubuntu alongside the install OS and the installer made the room for me.

Some folk don't try that and lose weeks or months trying to partition when if they had gone ahead and followed the directions they would have been done.
Bob

You've got to use a partitioning program to shrink the current partition. That leave x GB of unallocated space that the Ubuntu install will use to install Ubuntu on in a dual-boot configuration.

Just in case anything goes wrong during the shrinking, it makes sense to make an image-backup first. Most likely (99,9% or more) you won't need it, but in that 0,1 or 0,01% of the cases you lose everything. But that's your choice.

Kees

I used Easeus Partition Manager (free) to shrink the partition that Windows 7 (500 GB) was on to create two other partitions. I then downloaded Ubuntu and installed it to one of the newly created partitions. I am now able to dual-boot either Windows 7 or Ubuntu. Easeus was easy to use and Ubuntu did the rest. I lost nothing in shrinking the partition and had no problems in creating two others. It was easy and I am no computer whiz.